Evolution of Mesozoic-Cenozoic terrestrial ichnofaunas from southern South America.

GENISE, J. F.; MELCHOR, R. N.; BELLOSI, E.; SÁNCHEZ, M. V.; KRAUSE, J. M.; SARZETTI, L.; VERDE, M.; BEDATOU, E.

Cracovia, Polonia

Congreso; II International Congress on Ichnology; 2008

Jagiellonian University

Southern South America, and particularly Patagonia, contains a protracted record of continental deposits bearing diverse invertebrate ichnofossils. The study of these deposits, mostly composed by primary and recycled volcaniclastic materials, and ranging in age from Jurassic to Miocene, allows to trace the evolution of Mesozoic-Cenozoic terrestrial ichnofaunas. The Late Jurassic to Early Cretaceous Bajo Grande Formation and Baqueró Group contain similar ichnofabrics dominated by large vertical, lined “Y burrows (Loloichnus) and meniscate burrows of similar diameter attributed to crayfish producers. This ichnofabric may he overprinted by a boxwork of unlined, small burrows (sometimes meniscated), and rare chambers, which is assigned to the work of earthworms. A less common ichnofabric from these deposits are the first known crayfish nesting or recruitment trace fossils (Dagnichnus and Cellicalichnus). During the Late Cretaceous, as exemplified from the Bajo Barreal and Laguna Palacios Formation, the crayfish and earthworm ichnofabrics continue to be important and well developed. In addition, the first insect trace fossils appear, including pupal chambers (Rebuffoichnus, Pallichnus) and bee nests (Cellcalichnus). The K-T event produced a change from crayfish/earthworm dominated ichnofaunas to insect dominated ones. The late Paleocene-middle Eocene Rio Chico Group shows few trace fossils, probably due to taphonomic causes: Eatonichnus (brood structure of coprophagous beetles), Feoichnus (cicada feeding chambers), and rare occurrences of the earthworm ichnofabric. In contrast with the paucity of trace fossils in paleosols, contemporaneous early to middle Eocene floras from Laguna del Hunco and Rio Pichileufú display a large variety of insect traces in leaves. These include leaf-cutting, mining, galls, bite marks and oviposition traces, suggesting a large insect diversity that parallels the enormous diversity of plants. The probably coetaneous paleosols of the Asencio Formation from Uruguay (early Eocene) also display a large diversity of insect ichnofossils, essentially dominated by bee and coleopteran trace fossils (Coprinisphaera, Celliforma, Corimbatichnus, Ellipsoideichnus, Palmiraichnus, Teisseirei, Uruguay, Rebuffoichnus and Monesichnus). The producers of this ichnofauna, occurring at a lower latitude, probably reached Patagonia later, resulting in the burst of insect trace fossils from the middle Eocene-Miocene Sarmiento Formation and the early Miocene Pinturas Formation. Paleosols of these units contain a highly diversified ichnofauna with abundant insect trace fossils, especially brooding balls of dung beetles (Coprinisphaera). Other recorded trace fossils include Teisseirei, Celliforma, Pallichnus, Feoichnus, Lazaichnus and rare Loloichnus and earthworm traces. The dominance of Coprinisphaera in the Sarmiento Formation ichnofauna, along with other sedimentologic, paleopedologic and paleontologic evidence, records the appearance and evolution of one of the oldest (middle Eocene) open grasslands ecosystems worldwide.